This invention relates to the art of vacuum wheels and more particularly to a vacuum wheel vacuum opening configuration that has improved vacuum holding power to hold articles in place.
A vacuum wheel in the form of a rotary member having vacuum holes opening onto a cylindrical outer surface for the support and retention of stretchable film is typically a component of an apparatus that is known for various applications. A common example where an apparatus including a vacuum wheel would be used includes the construction of apparel that is worn on the body such as disposable diapers. In this application, an elastic waistband is stretched before being inserted into the waistband region. An example of such an apparatus is described in U.S. Pat. No. 4,925,520, commonly owned by the assignee hereof.
It is a common problem in such devices to experience insufficient vacuum holding strength for the materials to be held in place in relation to the shear forces applied to the materials. Another problem, where vacuum slots are used to improve the vacuum holding strength, is the loss of vacuum pressure along an edge of the vacuum slot. The vacuum holding force is a function of the area under the vacuum and the edges of the vacuum openings and slots against which the forces are applied. Simple round holes must be kept small in diameter to prevent the film from being sucked deep into the vacuum openings. The small area limits the holding force, and the small size limits the working edge length.
Various approaches have been taken for retaining flexible materials on a vacuum wheel. One approach has been to increase the number of vacuum openings on the available surface of the vacuum wheel. This can cause the size of the vacuum wheel to exceed possible size requirements for use in an apparatus. Another approach has been to include mechanical gripping means to engage the ends of the film to be stretched, such as pins or other rough surface features, in addition to the vacuum or drawing force on the article to be stretched. Although this approach improves the gripping of the article to the vacuum wheel, it does not allow the article to slip over the vacuum wheel when the shear force applied to the article exceeds the vacuum holding strength.
Because of the foregoing deficiencies in the art, an object of the present invention is to provide a compact vacuum wheel configuration that solves these problems by making the available vacuum wheel surface area more effective by improving its vacuum holding strength.